Due to advantages of easy maintenance, high bandwidth, and low cost, the PON, as an ideal physical platform in which various services, such as voice, data, and video, are accessed in an integrated manner through a single platform, becomes the best in optical access technologies. The PON technology is a Point to Multipoint (P2MP) optical fiber access technology. The PON is formed by an Optical Line Terminal (OLT), Optical Network Units (ONUs), and an Optical Distribution Network (ODN), and the advantages come from a passive optical splitter/coupler in the ODN, so that the PON does not need to use elements that have amplifying and relaying functions. The PON adopts a P2MP topology structure, so that the PON needs to adopt a P2MP multiple access protocol to enable the ONUs to share the OLT and the backbone optical fiber. It is agreed in the PON system that a direction of data from the OLT to the ONU is a downstream direction, and a direction from the ONU to the OLT is an upstream direction. Currently, in terms of the borne content, the PON may be divided into many types, where Gigabit-Capable Passive Optical Network (GPON) is relatively important. The GPON system adopts a Wavelength Division Multiplexing (WDM) technology, and realizes bi-directional transmission in a single fiber. In order to separate signals in come and go directions of a plurality of users on the same optical fiber, a downstream data stream adopts a broadcast technology; and an upstream data stream adopts a Time Division Multiple Access (TDMA) technology.
Transmission of the GPON in the upstream direction is in a burst mode. Each ONU sends data to the OLT according to a particular time slot allocated by the OLT. When receiving the data of each ONU, the OLT needs to first perform synchronization to acquire a start position of a data frame sent by the ONU, and then start to receive the data. In ITU-T G.984.3 standard, an upstream burst receiving synchronization mechanism is given. A preamble field and a delimiter field are set in the front of an upstream burst frame of the GPON. The OLT performs receiving synchronization of the upstream burst frame by using the two fields.
The preamble field is a string of binary sequence, which is convenient for a receiving end of the OLT to perform automatic gain control, clock recovery, and synchronous receiving. When receiving the upstream burst frame, the OLT enables the delimiter to match the received upstream burst frame. After success of matching, the OLT may learn the start position of the data in the burst frame, so as to finish the synchronization operation. In the solution of the prior art, six types of delimiter fields are defined, and the user may select one to perform configuration as required.
The error probability defined in the current GPON system is 10−4, and P(lost_burst)≦10−10. The delimiter given in the G984.3 standard can only satisfy the requirement of the existing GPON system. However, in the GPON system of the next generation, both upstream and downstream transmission rates need to be improved and are required to support full service, so that new requirements are brought to the bit error rate of the channels and the receiving end. It is quite difficult for the delimiter field that has the length of 16 bits or 20 bit and is given in the prior art to satisfy the requirement of the GPON system of the next generation, so that it is necessary to provide a new burst synchronization delimiter field and a method for sending an upstream transfer frame.